As is usually the case, when an oil well is first drilled, oil will often flow from the well under the natural pressure existing in the reservoir. When this natural pressure becomes insufficient, further quantities of oil may be recovered through the use of mechanical pumps. However, it is usually possible to recover only a small fraction of the total oil present in the formation by such so-called primary recovery methods. To recover oil beyond that produced by primary methods, a variety of supplemental production techniques have been employed. In these supplemental techniques, commonly referred to as secondary or tertiary recovery operations, a fluid is introduced into the oil-bearing formation in order to displace oil to a production system comprising one or more production wells. The displacing or "drive" fluid may be an aqueous liquid such as brine or fresh water, a gas such as carbon dioxide, steam or dense-phase carbon dioxide, an oil-miscible liquid such as butane, or an oil and water-miscible liquid such as an alcohol. Often, the most cost-effective and desirable secondary recovery methods involve the injection of steam. In practice, a number of injection and production wells will be used in a given field arranged in conventional patterns such as a line drive, a five spot or inverted five spot, or a seven spot or inverted seven spot.
In the use of the various flooding techniques, it has become a common expedient to add various polymeric thickening agents to the drive fluid to increase its viscosity to a point where it approaches that of the oil which is desired to be displaced, thus improving the displacement of oil from the formation. The polymers used for this purpose are often said to be used for "mobility" control.
Another problem encountered is that certain injected drive fluids may be much lighter than the reservoir fluids and thus separate by gravity, rising toward the top of the flowing region and resulting in the bypassing of the lower regions. This phenomena is known as gravity override.
Also encountered in the use of the various flooding techniques is a situation caused by the fact that different regions or strata often have different permeabilities. When this situation is encountered, the drive fluid may preferentially enter regions of higher permeability due to their lower resistance to flow rather than the regions of low permeability where significant volumes of oil often reside.
It therefore is often desirable to plug the regions of high permeability, or "thief" zones, either partly or entirely, so as to divert the drive fluid into regions of lower permeability. The mechanical isolation of these thief zones has been tried but vertical communication among reservoir strata often renders this method ineffective. Physical plugging of the high permeability regions by cements and solid slurries has also been tried with varying degrees of success; however, these techniques have the drawback that still productive sites may be permanently closed.
As a result of these earlier efforts, the desirability of designing a slug capable of sealing off the most permeable layers so that the drive fluid would be diverted to the underswept, "tighter" regions of the reservoir, became evident. This led to the use of oil/water emulsions, as well as gels and polymers for controlling the permeability of the formations. This process is frequently referred to as "flood conformance" or "profile control", a reference to the control of the vertical permeability profile of the reservoir. Profile control agents which have been proposed include oil/water emulsions, gels, e.g., lignosulfate gels and polymeric gels, with polymeric gels being the most extensively applied in recent years.
Among the polymers so far examined for improving flood conformance are polyacrylamides, polysaccharides, celluloses, furfural-alcohol and acrylic/epoxy resins, silicates and polyisocyanurates. A major part of this work has been conducted with the polyacrylamides, both in their normal, non-crosslinked form, as well as in the form of metal complexes, as described, for example, in U.S. Pat. Nos. 4,009,755, 4,069,869 and 4,413,680. In either form, the beneficial effects derived from these polyacrylamides seem to dissipate rapidly due to shear degradation during injection, low pH and high temperature. To overcome these problems and to achieve deeper polymer penetration into the reservoir, dilute solutions of these polymers have sometimes been injected first and then complexed in-situ.
Another group of polymeric thickeners which has received considerable attention for use in improving flooding are polysaccharides, particularly those produced by the action of bacteria of the genus Xanthomonas on carbohydrates. For example, U.S. Pat. Nos. 3,757,863 and 3,383,307 disclose a process for mobility control by the use of polysaccharides.
U.S. Pat. Nos. 3,741,307, 4,009,755 and 4,069,869 disclose the use of polysaccharides in the control of reservoir permeability. U.S. Pat. No. 4,413,680 describes the use of crosslinked polysaccharides for selective permeability control in oil reservoirs.
Another type of polysaccharide which has been experimented with in the area of profile control is the non-xanthan, heteropolysaccharide S-130. S-130 belongs to the group of welan gums and is produced by fermentation with a microorganism of the genus Alcaligenes. Another welan gum heteropolysaccharide, known as S-194, is also produced by fermentation with a microorganism of the genus Alcaligenes. U.S. Pat. No. 4,658,898 discloses the use of welan gum S-130 in saline waters. Additionally, crosslinking with organic compounds containing at least two positively charged nitrogen atoms is disclosed in U.S. Pat. No. 4,658,898; while Ser. No. 283,399, filed on Dec. 12, 1988, discloses welan gums crosslinked with phenolic resins or mixtures of phenols and aldehydes.
The use of various block copolymers for mobility control in flooding operations is described in U.S. Pat. Nos. 4,110,232, 4,120,801 and 4,222,881. Chung et al., U.S. Pat. No. 4,653,585, disclose the use of block copolymers, which may be crosslinked with polyvalent metal ions, for use as permeability control agents in enhanced oil recovery applications.
While a number of the different compositions discussed have been proposed for permeability control, some of these compositions under certain circumstances. For example, the polymers of Chung et al, may not be effectively crosslinked with polyvalent metal ions under all conditions encountered in the enhanced oil recovery applications, e.g., in acidic conditions. Polyacrylamides are known to display instability in the presence of high brine concentration or temperatures over 70.degree. C. Xanthan gums, while being very brine tolerant, display poor thermal stability, even at temperatures below 60.degree. C. Still, other polymers are unsuitable as permeability control agents when used in conjunction with steam flooding operations. This is due to the fact that they lose their structural integrity (i.e., they undergo "syneresis" or chemical decomposition) at the high temperatures generated during such operations.
Syneresis is the contraction or shrinking of a gel so that liquid is exuded at the gel surface. For example, a gel said to exhibit 20% syneresis would take up 80% of its original volume, with the remaining 20% being expelled water. Although the exact mechanism responsible for the syneresis of such gel-forming compositions is not fully understood, it is believed to result from the over-crosslinking of the polymeric material with time. While it is not yet known what an acceptable level of syneresis might be for profile control gels, it is believed that to minimize syneresis would enhance the effectiveness of such gels.
While low syneresis under high temperature reservoir conditions is one desirable feature of a polymeric gel for use in thermal enhanced oil recovery methods such as steam flooding, another beneficial feature is gel selectivity. As mentioned, selectivity is the ability to seek out and/or form a gel in the zones of higher permeability, rather than plug the zones of lower permeability where oil is likely to reside. Several means to impart selectivity to a gel or gel-forming composition are known. These include the use of shear-thinning, high viscosity polymeric gels which can be formed ex-situ pumped into the formation and subsequently reheal. Gels produced by the crosslinking of biopolymers often possess this ability, although their use in thermal enhanced oil recovery situations is suspect. Such polymeric solutions can then be complexed in-situ or formulated to crosslink slowly with time in the reservoir.
U.S. Pat. No. 4,074,757 discloses a method of selectively plugging the undesirable zones and channels of oil bearing reservoirs during high temperature oil recovery processes such as steam flooding, underground combustion flooding or a naturally occurring high temperature reservoir, or the like. Improved sweep efficiency is obtained by injecting a gel-forming solution consisting essentially of sodium or ammonium lignosulfonate and water or brine in the absence of other gelation promoters and then allowing the high temperatures of the underground formation to promote gelation. Other teaching of the use of polymers for injection as monomeric solutions to be subsequently polymerized in-situ include U.S. Pat. Nos. 4,461,351 and 4,637,467. The processes disclosed therein are not selective since the monomers can penetrate into the high as well as the low permeability zones of the formation before they polymerize and packers are required to inject the monomer into selected portions of the formation.
In view, therefore, there is a need for a method of selectively plugging highly permeable zones of a subterranean oil-bearing formation utilizing thermally-stable materials suitable for high temperature wells, steam flooded wells and the like.
It is, therefore, an object of the present invention to provide a process for selectively plugging highly permeable zones of a subterranean oil-bearing formation through the use of a polymeric gel-forming composition having good thermal stability under harsh conditions.
It is another object of the present invention to provide a process for the selective plugging of zones of high permeability within an oil-bearing formation when conditions of high temperature are encountered which utilizes a temperature selective gel-forming polymeric composition.
It is a further object of the present invention to provide a process for the selective plugging of zones of high permeability within an oil-bearing formation wherein the temperature at which the gelation reaction is activated may be adjusted in accordance with reservoir conditions.
It is yet another object of the present invention to provide a steam-flood enhanced oil recovery process which utilizes a high temperature stable gel-forming composition.
Other objects and the several advantages of the present invention will become apparent to those skilled in the art upon a reading of the specification and the claims appended thereto.